LAPACK  3.4.2
LAPACK: Linear Algebra PACKage
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dlaqr1.f File Reference

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Functions/Subroutines

subroutine dlaqr1 (N, H, LDH, SR1, SI1, SR2, SI2, V)
 DLAQR1 sets a scalar multiple of the first column of the product of 2-by-2 or 3-by-3 matrix H and specified shifts.

Function/Subroutine Documentation

subroutine dlaqr1 ( integer  N,
double precision, dimension( ldh, * )  H,
integer  LDH,
double precision  SR1,
double precision  SI1,
double precision  SR2,
double precision  SI2,
double precision, dimension( * )  V 
)

DLAQR1 sets a scalar multiple of the first column of the product of 2-by-2 or 3-by-3 matrix H and specified shifts.

Download DLAQR1 + dependencies [TGZ] [ZIP] [TXT]
Purpose:
      Given a 2-by-2 or 3-by-3 matrix H, DLAQR1 sets v to a
      scalar multiple of the first column of the product

      (*)  K = (H - (sr1 + i*si1)*I)*(H - (sr2 + i*si2)*I)

      scaling to avoid overflows and most underflows. It
      is assumed that either

              1) sr1 = sr2 and si1 = -si2
          or
              2) si1 = si2 = 0.

      This is useful for starting double implicit shift bulges
      in the QR algorithm.
Parameters:
[in]N
          N is integer
              Order of the matrix H. N must be either 2 or 3.
[in]H
          H is DOUBLE PRECISION array of dimension (LDH,N)
              The 2-by-2 or 3-by-3 matrix H in (*).
[in]LDH
          LDH is integer
              The leading dimension of H as declared in
              the calling procedure.  LDH.GE.N
[in]SR1
          SR1 is DOUBLE PRECISION
[in]SI1
          SI1 is DOUBLE PRECISION
[in]SR2
          SR2 is DOUBLE PRECISION
[in]SI2
          SI2 is DOUBLE PRECISION
              The shifts in (*).
[out]V
          V is DOUBLE PRECISION array of dimension N
              A scalar multiple of the first column of the
              matrix K in (*).
Author:
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date:
September 2012
Contributors:
Karen Braman and Ralph Byers, Department of Mathematics, University of Kansas, USA

Definition at line 122 of file dlaqr1.f.

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